The National Institute of Justice has released an article, published in Criminology & Public Policy, detailing the effects of school resource officers on school crime and responses to school crime. This study in this article, funded by a grant from the NIJ, examined schools that increased their spending on SROs.
You can read the article online at the Wiley Online Library at this link.
In the study linked above, evidence was found that SROs do not necessarily make schools safer, and generally end up increasing the criminalization of school discipline.
The presence of school resource officers in the classroom is a controversial topic. In Lincoln, the school board debated whether or not to continue its $500,000 contract with the Lincoln Police Department to have SROs in the schools. Members of the community and the school board said that the money would be spent better elsewhere, such as on social workers, counselors, school psychologists, and so on. The school board ultimately decided to keep the contract with LPD.